This invention applies to the fields of wafer and mask inspection, inspection review and diagnosis, measurement of critical circuit dimensions, low energy electron microscopy, emission microscopy and scanning microscopy. This disclosure describes several novel improvements to the LEEM and other tools that make such tools attractive for semiconductor inspection, review and CD metrology applications. These improvements allow imaging of partially or fully insulating substrates, which is a necessary requirement for these applications.
The low energy emission microscope (LEEM) is a specialized electron microscope that forms images of surfaces. Some examples of prior art LEEM systems are described in the review paper: xe2x80x9cThe continuing development of the low energy electron microscope for characterizing surfacesxe2x80x9d L. Veneklasen, Rev. Sci. Inst. 63(12) p. 5513, (December/1992) and in its references. The LEEM is a direct imaging (as opposed to scanning) microscope, more closely related to the transmission electron microscope than the scanning electron microscope. The LEEM includes a cathode lens whose negative electrode is an electrically conducting substrate surface. Images are formed from low energy electrons that are emitted, scattered or reflected from many points on the flat surface. The cathode lens accelerates and focuses these electrons to form an image. The image can be further magnified by a series of projection lenses acting upon the accelerated beam, and then recorded upon a scintillator screen viewed by a television camera. Resolution can be in the range of 5-100 nm, depending upon the field strength at the surface and the aperture angle used for imaging, as described in xe2x80x9cComparing cathode lens configurations for low energy electron microscopyxe2x80x9d, J. Chmelik, L. Veneklasen and G. Marx, OPTIK 85:5 (1989) 155-60J.
A simplified prior art LEEM configuration is shown in FIG. 1. Prior art LEEMs use a single illumination beam 1 which is accelerated to about 10 to 30 keV in an electron gun 2. The beam passes through a separator magnet 3 that bends the beam into the axis of the cathode lens 4. An image of the gun crossover is transferred to the back focal (diffraction) plane 5 of the cathode lens, forming a parallel flood beam 6 that uniformly illuminates the substrate 7. The substrate is electrically floated at approximately the same voltage as the cathode of the electron gun, so that illuminating electrons are decelerated in the cathode lens, striking the substrate 7 at energies between 0 to about 1000 eV.
FIG. 1 shows the details of electron paths in the cathode lens. There is a high electric field at the substrate surface, so electrons leaving the substrate follow a parabolic path as they are re-accelerated in the lower part of the cathode lens. After passing through the lens, they form a crossover 9 at the back focal plane of the cathode lens 4. This distribution reflects the angular distribution of emission and reflection from the substrate. An aperture placed at this crossover point 9, or at its conjugate points 10 further down the beam path, can be used to obtain bright field image contrast by excluding from the image those electrons that have scattered or reflected at larger angles. Alternately, an offset or annular aperture may also be used for dark field imaging that only includes those electrons that have scattered or reflected at larger angles. After emerging from the cathode lens, the image beam 11 is bent in the opposite direction by the separator magnet 3. Additional projection lenses 12 and 13 further magnify the image before it strikes a scintillator screen 14 that is viewed by a TV camera 15.
Conventional LEEMs offer several imaging modes. When the voltage (surface potential) of the substrate surface is slightly more negative than the cathode, electrons turn around just before reaching the surface. Surface topography causes lateral electric fields near the substrate, which deflect low energy electrons in various directions; some following paths that do not pass through the contrast aperture in the imaging path. Since all illuminating electrons are reflected, this mirror imaging mode creates intense, high contrast images of substrate topography.
More positive substrate bias (5 to 30 eV landing energy) favors elastic backscatter images, where illumination electrons scatter from the surface without losing energy. Even more positive substrate bias (30 to 800 eV landing energy) generate secondary electron images, where different electrons are ejected from the surface with a wide angular distribution and energy spread. These images are less intense because many of the scattered electrons cannot pass through the contrast aperture.
Most prior art wafer and mask inspection systems use light optical images. Scanning electron beam microscopes (SEMs) have also been developed for inspection and CD measurement. Instead of forming a full field image, these SEM instruments scan a very small beam over the surface, and record the re-emitted secondary electrons in a single detector. Image acquisition tends to be slower than direct imaging light optical instruments because only one image element (pixel) at a time is recorded.
Prior art LEEMs can only image electrically conductive substrates. Imaging of semiconductor circuits poses a new challenge, for semiconductor wafers and chrome glass masks have insulating as well as conducting layers. A typical circuit consists of metal conductors, doped silicon, polysilicon, and oxide insulating regions arranged as continuous and isolated features on several layers. Some conducting traces and all insulating surfaces are not electrically connected to a grounded substrate layer.
Insulating surfaces are not a problem for light optical inspection because the scattering and reflection of light is insensitive to electrostatic charge on the surface. However, both scanning and direct imaging electron beam instruments exhibit charging effects. The rate that a given pixel element charges depends upon the difference between electron flux arriving at and leaving each pixel. The high current densities required for imaging at inspection rates imply a high rate of charging if the electron flux leaving the surface is not exactly balanced by that entering. Thus, the surface voltage can quickly reach levels detrimental to imaging or even sample integrity. Means of controlling local surface charging are therefore needed if electron microscopes are to be used for inspection of wafers and masks.
Prior art methods of controlling charging in scanning electron microscopes often involve the use of moderate energy illumination beams. There is usually a beam energy where the average charge leaving the substrate equals the average charge absorbed in the substrate. Various electrode configurations have been developed that allow some of the secondary electrons to return to the substrate to maintain charge balance, and are described in commonly assigned U.S. Pat. No. 5,502,306, which is incorporated by reference as though fully set forth herein. Side mounted ion and electron flood guns, and jets that supply gas to be ionized have also been used to supply clouds of very low energy charge to discharge the substrate. These methods can reduce the overall charging, but do little to eliminate the charging in the neighborhood of the scanned beam, which has much higher current density than the flood beam.
It is difficult to form mirror images of insulating surfaces in a prior art LEEM using a single low energy beam. An insulating surface is illuminated by a xe2x80x9clow energyxe2x80x9d electron beam, incident perpendicular to the surface with essentially low landing energy ( less than xc2x10.5 eV). This energy is too low to cause secondary emission, so the electrons may be either absorbed or reflected. Absorbed electrons charge the surface more negatively, causing more of the beam to be reflected. A balance is established where the surface is just barely negative enough to for leakage current to balance absorption. The surface potential can vary only within the range of energies in the illuminating beam.
Without a second higher energy beam, the mirror image from insulating surfaces is not very useful. When a surface is highly insulating, the leakage current is very low, and the surface still charges to a fairly high negative potential because the beam contains a very small fraction of electrons at a significantly higher energy than the average. This negative charging prevents most of the electrons from coming close to the surface, which in turn prevents high resolution mirror imaging from insulating areas. A second beam is needed to supply some positive charge to the insulating surface to keep it near the substrate potential.
It is also difficult to form backscatter or secondary electron images of insulating surfaces in a prior art LEEM using a single higher energy beam. The insulating surface is illuminated by a higher energy beam. Electrons may be absorbed or scattered, but their energy is too high for any to be reflected back before hitting the surface. When more electrons are absorbed than scattered, the surface charges negatively. When more are scattered (re-emitted) than are absorbed, the surface charges positively because each secondary electron leaves behind a positive charge. The surface will continue to charge until the insulating layer breaks down or until its surface potential adjusts to result in a landing energy that balances the leakage, absorption and scattering fluxes. The charging rate depends upon the difference in flux divided by the capacitance of the surface layer.
This invention involves a method and/or apparatus for imaging a substrate, comprising exposing the substrate to a first set of electrons, the first set of electrons having an energy selected to maintain surface voltage present on the substrate at a predetermined level and causing a fourth set of electrons to leave the substrate; exposing the substrate to a second set of electrons, the second set of electrons having an energy selected to cause a third set of electrons to leave said substrate, and detecting at least one of the third set or fourth set of electrons, thereby imaging a portion of the substrate.